26 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



Fig. 27 Yoldiella bilanta. Lateral view of the internal morphology 

 from the right side of a specimen from Sta. 192 Cape Basin. 

 (Scale 1.0 mm). For identification of parts see Fig. 5. 



Shell description (Fig. 28). Shell small, ovate, not 

 inflated, inequilateral (posterior umbonal length 54-58% of 

 total length), in larger specimens antero-ventrally, very fine 

 concentric lines form broadly spaced ridges, periostracum 

 pale yellow, umbo slightly raised, inwardly directed; dorsal 

 margin slightly convex, antero-dorsal margin curves moder- 

 ately steeply and evenly with anterior margin, ventral margin 

 smooth curve, postero-dorsal margin slopes gently from 

 umbo to extended posterior margin; ligament amphidetic, 

 small, rounded, not extending below hinge plate, with small 

 secondary anterior and posterior external extensions of fused 

 periostracum ; hinge plate moderately long, relatively nar- 

 row, small chevron-shaped teeth, 7 anterior and 8 posterior. 



Prodissoconch length: 200 p.m. Maximum recorded shell 

 length: 2.78 mm. 



Shell measurements (mm) & ratios are as follows:- 



Length 



Height 



Width 



H/L 



W/L 



PL/TL 



2.00 



1.46 





0.73 





0.54 



2.69 



1.89 



- 



0.70 



- 



0.56 



2.78 



1.98 



- 



0.71 



- 



0.58 



1.64 



1.13 



0.53 



0.69 



0.33 



0.54 



2.23 



1.55 



0.84 



0.70 



0.38 



0.57 



2.60 



1.80 



1.01 



0.69 



0.38 



0.55 



2.52 



1.76 



1.01 



0.70 



0.40 



0.57 



PL = postumbonal length 



Internal morphology (Fig. 29). The combined exhalent 

 and inhalent siphons are large. The inhalent is somewhat 

 shorter than the exhalent and open ventrally. A large sipho- 

 nal tentacle originates on the left side of the siphonal embay- 

 ment close to the base of the siphon. The feeding aperture is 

 not particularly well-developed. There is a large anterior 

 sense organ. The posterior adductor muscle is long and 

 narrow, and only half the size of the bean-shaped anterior 

 muscle. The gills are well-developed with up to 20 gill plates. 

 The labial palps are relatively small with up to 12 internal 

 palp ridges with long, thin palp proboscides. The foot is 

 extremely long and slender with an extended sole fringed 

 with deep papillae. There is a large byssal gland in the heel 

 with large pedal ganglia dorsal to it. The visceral and cerebral 

 ganglia are of moderate size and club-shaped. The hind gut is 

 similar to that in Yoldiella lata (p. 32) with one complete turn 

 of a double loop to the right side. That in Y. artipica entends 

 further posteriorly than that in Y. lata. As in many yoldiellid 

 species the body wall enclosing the hind gut loops overhangs 

 the palps on the right side (Fig. 29). 



Yoldiella similis (new species) 



Type locality. R.V. Atlantis II, Cruise 42, Sta. 197, 

 Angola Basin, 21.5.1968, 10°24'S, 9°09'E - 10°29'S, 9°04'E, 

 Epibenthic Trawl, 4559-4566 m. 



Fig. 28 Yoldiella artipica. A dorsal and two right 

 lateral views of shells, and one lateral view of a left 

 valve to show detail of hinge-plate. Specimens from Sta. 

 200 Angola Basin. (Scale = 1.0 mm). 



